This invention relates to an electrically insulated needle tip apparatus for producing an electrospray formed from a sample solution. More particularly this invention relates to an electrically insulated needle tip apparatus for forming an electrospray which is converted to an ionic stream for analysis.
A liquid flowing through a capillary jet or orifice may be converted to a spray of small charged droplets (of the order of 1 .mu.m is diameter) by applying a strong electric field to the liquid as it merges from the tip of the capillary. For a sufficiently high applied field, the electrostatic stress imposed by the field and the surface-induced electrical charge is sufficient to overcome the surface tension forces on the liquid. Breaking apart into a large number of small charged droplets is a way for the liquid to disperse the charge and reach a lower total energy state. This process of forming a spray is commonly known as electrospray.
At the present time apparatus are available for forming an electrospray of a sample solution such as a liquid stream effluent from a liquid chromatography separation step and subsequently analyzing the electrospray with a mass analyzer such as a quadrupole mass spectrometer, an ion trap mass spectrometer, a timeof-flight mass spectrometer or a magnetic sector mass spectrometer or the like.
In a liquid chromatograph, a stream of solvent, containing a mixture of chemical species in solution, is passed by elevated pressure through a chromatographic column. The column is so designed that it separates the mixture, by differential retention on the column, into its component species. The different species then emerge from the column as distinct bands in the solvent stream, separated in time. Coupling the output of a liquid chromatograph to a mass spectrometer via an electrospray interface gives the analyst a powerful tool since it can provide molecular weight and structural information about the separated species as they emerge from the liquid chromatograph.
At the present time, electrically conductive needles, typically formed of stainless steel are utilized for passage of a liquid sample and from which emerges an electrospray under the appropriate electrical field conditions. Needles of similar construction are utilized in an ion spray process which is a form of an electrospray process in which the liquid is nebulized by a turbulent flow of gas such as nitrogen. The use of electrically conductive needles permits only a narrow voltage operating range. In order to obtain a sufficiently high field strength to produce an electrospray, a minimum voltage bias, typically about 2.0 to 2.5 kilovolts (kv) must be applied to the needle. The value of the minimum voltage will vary with hardware configuration and solvent properties of the sample solution such as surface tension, polarity and viscosity. Likewise, there exists a maximum voltage bias, typically about 3.0 to 4.0 kv, beyond which poor or no spectra is obtained from downstream analytical apparatus. This is due to dissociative ionization of the target solute compound due to corona discharge at the needle tip when a sufficiently high voltage which produces the corona discharge is utilized. Alternatively, when the distance between the two electrodes is too small, undesirable arcing occurs between the needle and the downstream counterelectrode which results in breakdown of the electrospray produced. Both of the conditions limit the electrical field that can be utilized to form the electrospray.
It has been proposed by Bruins et al, Anal. Chem., Vol 59, pp 2642-2646 (1987) to alleviate these problems in a pneumatically assisted electrospray system by flushing the needle tip area with gas or gas mixtures which are arc and corona suppressants. This apparatus is sufficiently complex as to require the user to empirically determine optimum condition of gas flow, solvent flow and needle voltage for each sample processed.
Ikonomou et al, Anal. Chem., Vol 63, pp 1989-1991 discloses a needle construction utilized in electrospray or ion spray apparatus. The needle construction comprises an inner silica capillary surrounded by a stainless steel capillary. The solution being converted to an electrospray is passed through the silica capillary. An inert gas such as nitrogen can be passed within a cylindrical space between the silica capillary and the stainless steel capillary. The exposed stainless steel capillary functions as the electrode.
Smithy et al, Anal. Chem. Vol 60, pp 436-441, 1998 discloses a needle construction for an electrospray apparatus similar to that of Ikonomou et al and which includes a silver coating on the outside surface of the stainless steel capillary.
Smith et al, Anal. Chem, Vol 60, pp 1948-1952, 1988 discloses an electrospray needle construction comprising an inner silica capillary surrounded by and spaced apart from an outer stainless steel capillary. An electrically conductive buffer solution is passed between the inner capillary and the outer capillary to form a sheath liquid surrounding the liquid sample which passes through the inner silica capillary.
In all of these needle constructions, the steel capillary is exposed at or near the exit end of the needle construction. The steel capillary functions as the upstream electrode from which the electrical field emanates in cooperation with a downstream counterelectrode. The use of an exposed upstream electrode exposed to the atmosphere severely limits the electrical potential that can be applied thereto without causing undesirable arcing. This limitation, in turn limits the energy that can be applied to the liquid sample emanating from the needle and thereby limits the flow rate of liquid that can be passed through the needle.
It would also be desirable to provide a means for increasing liquid sample flow rate through the needle so that the adverse effect of leakage in the apparatus effecting liquid sample flow would be minimal as compared to the flow rate of liquid sample.
Accordingly, it would be desirable to provide a means which permits utilizing electric fields of greater intensity to form an electrospray that can be utilized with presently available apparatus. The use of such higher intensity fields would permit higher flow rates of sample solution to be processed as compared to flow rates that can be processed with presently available apparatus. In addition, it would be desirable to provide such an apparatus which eliminates the possibility of corona formation.